Atomic clock technology is well known and mature, with several companies marketing primary and lab-grade “secondary” atomic clocks. A definition of a primary frequency standard is those atomic-based clocks whose output frequency is equal to the free atom frequency, the frequency typically oscillating on the classic “0-0” clock transition without shifts. A working, practical, definition of a primary standard is that class of frequency standards whose output frequency is known without requiring calibration after fabrication and which have minimal long-term drift. Cesium beam clocks are one such technology of primary frequency standards. As opposed to traditional rubidium clocks which utilize buffer-gas vapor cells (and not considered primary standards), cesium beam clocks operate by interrogating a beam of alkali-atoms traveling within a vacuum enclosed Ramsey cavity, eliminating clock perturbations due to buffer-gas and wall collisions and eliminating need for clock calibration after manufacture.
Vapor cell clocks utilize a buffer-gas background to mitigate against alkali-collisions with cell walls, an event that destroys the atomic spin coherence. The buffer gas minimizes the effects of wall collisions and provides for long atom interrogation times. However, the buffer gas species can cause buffer-gas pressure shifts itself to the fundamental clock hyperfine frequency, causing frequency shifts and hence accuracy errors noticeable in clock-to-clock builds as well as frequency aging over time due to long-term changes in buffer-gas composition and pressure over time.
Traditional vapor-cell clocks are therefore not considered primary standards as it is not possible to fabricate each vapor cell in a manufacturing line to contain the exact same number of buffer-gas molecules and mixture. Currently, these clocks are made with known buffer-gas mixtures that result in zero effect on the temperature-induced frequency shift. Such a temperature-invariant buffer-gas mixture yields practical benefits allowing the vapor cell to experience changes in it's temperature without causing clock shift (as changes in the ambient temperature will cause some slight change in cell temperature), but results in a finite pressure-induced 0-0 frequency shift. These vapor-cell clocks are calibrated after manufacture.
Similarly, there is a buffer-gas mixture for operation at the zero pressure-shift point. Such a mixture has a finite residual temperature coefficient, so for practical systems would require exquisite control on the operating temperature of the alkali-vapor cell to prevent clock errors due to changes in the ambient temperature, a feat not readily feasible due to limitations of standard heater/temperature sensor systems.
Thus, there is a need in the art to provide a buffer gas clock, and method for fabricating the same, that operates as a primary frequency standard; i.e., operating at the free-atom frequency with no clock frequency shifts, without long-term frequency drifts and without need for calibration after manufacture.